1. Field of the Invention
The present invention relates to a process for the long-term operation of a heterogeneously catalyzed gas phase partial oxidation of propene to acrylic acid, by initially conducting a starting reaction gas mixture 1 which comprises propene, molecular oxygen and at least one inert gas and contains the molecular oxygen and the propene in a molar O2:C3H6 ratio of ≧1, in a first reaction stage, at elevated temperature, over a first fixed catalyst bed 1 whose catalysts are such that their active composition is at least one multimetal oxide containing molybdenum and/or tungsten and also at least one of the elements bismuth, tellurium, antimony, tin and copper, in such a way that the propene conversion in single pass is ≧93 mol % and the accompanying selectivity of acrolein formation and of acrylic acid by-production taken together are ≧90 mol %, optionally reducing the temperature of the product gas mixture 1 leaving the first reaction stage by direct and/or indirect cooling and optionally adding molecular oxygen and/or inert gas to the product gas mixture 1, and then conducting the product gas mixture 1, as a starting reaction gas mixture 2 which comprises acrolein, molecular oxygen and at least one inert gas and contains the molecular oxygen and the acrolein in a molar O2:C3H4O ratio of ≧0.5, in a second reaction stage, at elevated temperature, over a second fixed catalyst bed 2 whose catalysts are such that their active composition is at least one multimetal oxide containing the elements molybdenum and vanadium, in such a way that the acrolein conversion in single pass is ≧90 mol % and the selectivity of acrylic acid formation assessed over both reaction stages, based on propene converted, is ≧80 mol %, and by, in order to counteract the deactivation both of the fixed catalyst bed 1 and of the fixed catalyst bed 2, independently increasing the temperature of the particular fixed catalyst bed over time.
2. Description of the Background
Acrylic acid is an important monomer which finds use as such or in the form of its alkyl esters for obtaining polymers which are suitable, for example, as adhesives.
It is known that acrylic acid can be prepared by two-stage heterogeneously catalyzed gas phase partial oxidation of propene to acrylic acid in a fixed catalyst bed (cf., for example, EP-A 1159247, DE-A 10313208, DE-A 19948248, EP-A 990636, EP-A 1106598, DE-A 3002829). In the first reaction stage, propene is substantially partially oxidized to acrolein and, in the second reaction stage, the acrolein formed in the first reaction stage is substantially partially oxidized to acrylic acid. It is significant that the industrial embodiment is normally configured in such a way that the acrolein formed in the first reaction stage is not removed, but rather conducted into the second reaction stage as a constituent of the product gas mixture leaving the first reaction stage, optionally supplemented by molecular oxygen and inert gas, and optionally cooled by direct and/or indirect cooling. The fixed catalyst bed used in the particular reaction stage is tailored to the particular reaction stage and different to the fixed catalyst bed used for the other reaction stage. In addition, the two reaction stages are heated independently in such a way that the particular fixed catalyst bed is at its optimum working temperature.
The particular features of the two reaction stages are known per se (cf., for example, EP-A 714 700, EP-A 700 893, EP-A 279 374, EP-A 575 897, etc.).
It is also known that such a two-stage process for heterogeneously catalyzed gas phase partial oxidation of propene to acrylic acid may be operated substantially continuously over prolonged periods over one and the same fixed catalyst beds. However, the fixed catalyst beds lose quality in the course of the operating time. In general, both their activity and the selectivity of the particular target product formation deteriorate in both stages.
In order, despite this, to operate the fixed catalyst beds, whose manufacture and exchange is comparatively inconvenient and costly, for as long as possible in a reactor system charged with them, the prior art attempts in highly differing ways to counteract their aging process.
EP-A 990 636 (for example page 8, lines 13 to 15), EP-A 1070700 (for example page 4, lines 49, 50) and EP-A 1 106 598 (for example page 13, lines 43 to 45) propose the substantial compensation of the reduction in the quality of the particular fixed catalyst bed by gradually increasing the temperature of the particular fixed catalyst bed in the course of the operating time under otherwise substantially constant operating conditions, in order to substantially retain the propene or acrolein conversion in single pass of the reaction gas mixture through the fixed catalyst beds.
In this context, the temperature of the fixed catalyst bed refers to the temperature of the fixed catalyst bed when the partial oxidation process is performed, except in the theoretical absence of a chemical reaction (i.e. without the influence of the heat of reaction). This also applies in this document. In contrast, effective temperature of the particular fixed catalyst bed refers in this document to the actual temperature of the fixed catalyst bed taking into account the heat of reaction of the partial oxidation. When the temperature of a fixed catalyst bed is not constant along the fixed catalyst bed (for example in the case of a plurality of temperature zones), the term temperature of the fixed catalyst bed in this document means the (numerical) average of the temperature along the fixed catalyst bed.
It is significant in the aforementioned context that the temperature of the reaction gas mixture (and thus also the effective temperature of a fixed catalyst bed) passes through a maximum value (known as the hotspot value) when it passes through a fixed catalyst bed. The difference between hotspot value and the temperature of the fixed catalyst bed at the location of the hotspot value is referred to as the hotspot expansion.
A disadvantage of the procedure recommended in EP-A 990 636 and in EP-A 1 106 598 is that, with increasing increase in the temperature of the fixed catalyst bed, its aging process is accelerated (for example certain movement processes within the active compositions of the catalysts which contribute to aging proceed more rapidly). This is in particular because hotspot expansion usually rises more steeply than the temperature of the fixed catalyst bed itself with an increase in the temperature of the fixed catalyst bed (cf., for example, page 12, lines 45 to 48 of EP-A 1 106 598 and page 8, lines 11 to 15 of EP-A 990 636). The effective temperature of the fixed catalyst bed therefore usually increases disproportionately in the hotspot region, which additionally promotes the aging of the fixed catalyst bed.
When a maximum value of the temperature of the fixed catalyst bed is attained, the fixed catalyst bed is therefore customarily fully exchanged.
However, a disadvantage of such a complete exchange is that it is comparatively costly and inconvenient. The process for preparing acrylic acid has to be interrupted for a prolonged time and the costs of catalyst preparation are likewise considerable.
Operating modes are therefore desired for processes for two-stage heterogeneously catalyzed gas phase partial oxidation of propene to acrylic acid which are helpful in further prolonging the on-stream time of the fixed catalyst beds in the reactor system.
In this regard, DE-A 102 32 748 recommends, instead of fully exchanging the fixed catalyst bed, only replacing a portion thereof with a fresh catalyst charge.
A disadvantage of this proposal is that even a partial change of the fixed catalyst bed is accompanied by significant cost and inconvenience.
EP-A 614 872, which relates in particular to the second reaction stage in which the acrolein is partially oxidized to acrylic acid, recommends extending the on-stream time of a second stage fixed catalyst bed by, after operating the fixed catalyst bed for several years, which is accompanied by increases in the temperature thereof of from 15° C. to 30° C. and more, interrupting the process for partial oxidation, and, at fixed catalyst bed temperatures of from 260 to 450° C., conducting a gas mixture composed of oxygen, steam and inert gas through it, and subsequently continuing the partial oxidation.
In this context, inert gases in a gas mixture which is conducted through a fixed catalyst bed under certain conditions refers in this document to those gases of which at least 95 mol %, preferably at least 98 mol %, most preferably at least 99 mol % or 99.5 mol %, remain unchanged when they are conducted through the fixed catalyst bed. Regarding the gas mixture G to be used in accordance with the invention, steam should not be included under the term inert gas.
EP-A 169 449, which relates substantially to the first reaction stage in which the propene is partially oxidized to acrolein, recommends extending the on-stream time of the fixed catalyst bed by, after operating the fixed catalyst bed for several years, which is accompanied by increases in the temperature thereof of 15° C. and more, interrupting the process for partial oxidation, and, at fixed catalyst bed temperatures of from 380 to 540° C., conducting a gas consisting substantially of air through it, and subsequently continuing the partial oxidation.
EP-A 339 119 recommends, with an analogous mode of operation, the use of a gas comprising oxygen and steam.
However, a disadvantage of the procedures of EP-A 339 119, EP-A 169 449 and of EP-A 614 872 is that, up to the point at which the partial oxidation is interrupted, the aging of the fixed catalyst bed continues and is promoted unhindered.
It is an object of the present invention to provide a process for long-term operation of a two-stage heterogeneously catalyzed gas phase partial oxidation of propene to acrylic acid, in which the catalyst aging is counteracted in both stages in a manner by which the intensity of the hotspot expansion in both stages over time is lower than in the prior art processes.